NAME
clock_getres, clock_gettime, clock_settime - clock and time functions
SYNOPSIS
#include <time.h>
int clock_getres(clockid_t clk_id, struct timespec *res);
int clock_gettime(clockid_t clk_id, struct timespec *tp);
int clock_settime(clockid_t clk_id, const struct timespec *tp);
Link with -lrt.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
clock_getres(), clock_gettime(), clock_settime():
_POSIX_C_SOURCE >= 199309L
DESCRIPTION
The function clock_getres() finds the resolution (precision) of the
specified clock clk_id, and, if res is non-NULL, stores it in the
struct timespec pointed to by res. The resolution of clocks depends on
the implementation and cannot be configured by a particular process.
If the time value pointed to by the argument tp of clock_settime() is
not a multiple of res, then it is truncated to a multiple of res.
The functions clock_gettime() and clock_settime() retrieve and set the
time of the specified clock clk_id.
The res and tp arguments are timespec structures, as specified in
<time.h>:
struct timespec {
time_t tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
The clk_id argument is the identifier of the particular clock on which
to act. A clock may be system-wide and hence visible for all
processes, or per-process if it measures time only within a single
process.
All implementations support the system-wide real-time clock, which is
identified by CLOCK_REALTIME. Its time represents seconds and
nanoseconds since the Epoch. When its time is changed, timers for a
relative interval are unaffected, but timers for an absolute point in
time are affected.
More clocks may be implemented. The interpretation of the
corresponding time values and the effect on timers is unspecified.
Sufficiently recent versions of glibc and the Linux kernel support the
following clocks:
CLOCK_REALTIME
System-wide real-time clock. Setting this clock requires
appropriate privileges.
CLOCK_MONOTONIC
Clock that cannot be set and represents monotonic time since
some unspecified starting point.
CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
Similar to CLOCK_MONOTONIC, but provides access to a raw
hardware-based time that is not subject to NTP adjustments.
CLOCK_PROCESS_CPUTIME_ID
High-resolution per-process timer from the CPU.
CLOCK_THREAD_CPUTIME_ID
Thread-specific CPU-time clock.
RETURN VALUE
clock_gettime(), clock_settime() and clock_getres() return 0 for
success, or -1 for failure (in which case errno is set appropriately).
ERRORS
EFAULT tp points outside the accessible address space.
EINVAL The clk_id specified is not supported on this system.
EPERM clock_settime() does not have permission to set the clock
indicated.
CONFORMING TO
SUSv2, POSIX.1-2001.
AVAILABILITY
On POSIX systems on which these functions are available, the symbol
_POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The
symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME
indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID,
CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)
NOTES
Note for SMP systems
The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks are
realized on many platforms using timers from the CPUs (TSC on i386,
AR.ITC on Itanium). These registers may differ between CPUs and as a
consequence these clocks may return bogus results if a process is
migrated to another CPU.
If the CPUs in an SMP system have different clock sources then there is
no way to maintain a correlation between the timer registers since each
CPU will run at a slightly different frequency. If that is the case
then clock_getcpuclockid(0) will return ENOENT to signify this
condition. The two clocks will then only be useful if it can be
ensured that a process stays on a certain CPU.
The processors in an SMP system do not start all at exactly the same
time and therefore the timer registers are typically running at an
offset. Some architectures include code that attempts to limit these
offsets on bootup. However, the code cannot guarantee to accurately
tune the offsets. Glibc contains no provisions to deal with these
offsets (unlike the Linux Kernel). Typically these offsets are small
and therefore the effects may be negligible in most cases.
BUGS
According to POSIX.1-2001, a process with "appropriate privileges" may
set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks
using clock_settime(). On Linux, these clocks are not settable (i.e.,
no process has "appropriate privileges").
SEE ALSO
date(1), adjtimex(2), gettimeofday(2), settimeofday(2), time(2),
clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3),
sysconf(3), time(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
2010-02-03 CLOCK_GETRES(2)